Typically a laser diode is arranged in a PNP type circuit configuration or, alternatively, in an NPN type circuit configuration. The control of the supply voltage of the laser diode needs to be adapted to the type of circuit configuration which is used, otherwise the lifetime of the laser diode is reduced or the laser diode is even destroyed when the control of the power supply is initialized.
U.S. Pat. No. 6,363,044 B1 shows a photo diode detection circuit which is used to automatically identify a type of photo diode circuit connected to it. Once the type of photo diode circuit has been identified by applying a current to it and evaluating a voltage occurring at an input of the detection circuit, the photo detection circuit adapts itself to it, e.g. a reverse biasing voltage is or is not applied to the photo diode of the photo diode circuit. The result is a photo diode detection circuit which uses a forward or a reversed biased photo diode. The photo diode detection circuit is now in a state to measure light falling on the photo diode. The detection circuit is used for monitoring the light intensity of a light source and for a controlling circuit which enables to regulate the intensity of the light source. It is a disadvantage of this prior art controlling circuit that it does not automatically adapt to the circuit configuration in which the light source is arranged.